1. Field of the Invention
This invention relates to a method for controlling the force with which the peripheral part of a die is retained in an extruder.
2. Description of the Prior Art
In the conventional extruders, the extruder constructed as shown in FIG. 1, for example, has been renowned. This extruder 1 comprises a platen 2, a pair of container cylinders 8a and 8b fitted into depressed parts 2a and 2b of the platen 2 respectively, and a container 15. One end of each of piston rods 13a and 13b of pistons 14a and 14b slidably inserted in the container cylinders 8a and 8b respectively is fixed to the container 15, a die 7 is attached through the medium of a sub-bolster 3, a bolster 4, a backer 5, and a die ring 6 to the platen 2, and the die 7 is consequently retained by causing the container cylinders 8a and 8b attached to the platen 2 to press the container 15 through the medium of a spreader 16 to the die 7. A billet 17 accommodated in the container 15 is pressed by a stem or ram 19 through the medium of a dummy block 18 and then extruded through an orifice 7a of the die 7 to be converted into an extruded article of a prescribed shape.
In the extruder of this kind, for the purpose of precluding possible leakage of the billet 17 through the contact surface between the inner circumferential edge part 15a of the container 15 and the spreader 16, namely the container seal surface, in the process of extrusion, the container seal surface is sealed by causing the container 15 to be moved toward the platen 2 and pressed against the spreader 16 by means of the container cylinders 8a and 8b and the peripheral part of the die 7 is retained by pressing the die 7 against the backer 5 with the sealing force censequently produced. The force which retains the die 7 inside the extruder equals the sealing force mentioned above and this sealing force is fixed by the propelling force of the container cylinders 8a and 8b, namely by the pressure of fluid supplied to chambers 9a and 9b and the area exposed to the pressure. The extrusion is carried out while the sealing force mentioned above is maintained at a constant magnitude by virtue of the size of the die 7 or the pressing force of the stem 19.
When section articles of aluminum were produced by extruding aluminum ingots by the use of this extruder and then tested to find their section thicknesses, the results of the test indicate that the section articles sequentially emanating from the extruder showed a gradual decrease in section thickness from the initial stage of extrusion onward and an abrupt decrease in section thickness immediately before the butt end and that the amount of variation in section thickness was in the approximate range of 0.1 to 0.12 mm.